1967 Mercury 500 prop question

[1973Chieftain]

I recently bought a '67 Mercury 500 from another member who thought the prop was original... at least he has been using it for about 9 years. I tried ot on my 16' and did not like the proformance so I pulled the prop off. According to its numbers, its a 13x13... I counted 15 splines. Now from what I can find, the 500's of that era had 11 tooth splines and the 650's had 15 tooth ones... also, that the 13x13 was a 65 hp prop. All of the 11 tooth ones I could find for the 500 are 10.75x up yo 15". Does anyone know what OEM was? Does my engine have a shaft and prop from a 650? Im sorry, I don't have any rpm info as I font have a tach on it... would any of you prop experts be able to guess what would be the fastest size/ recommend a 15 tooth splined prop that is not up by 13" diameter? I was guessing something along the lines of a 10x17. Thanks in advance for amy suggestions!!!

 

[steelespike]

Re: 1967 Mercury 500 prop question

So is the motor a 15 spline shaft? and did the 13 diameter fit the motor.
My guess is you have had a leg change or something. Don't worry about the diameter you do very likely need a lower pitch.
It appears you may have a homemade Merc bigfoot.
If you tell us what you don't like and give us the details of your boat and performance.
A Tinytach about 47 dollars will work on almost any motor and a gps will get you your speed.
We can help you get setup.

 

[1973Chieftain]

Re: 1967 Mercury 500 prop question

Thanks Steel! A tach is on the list of things to do. What I did not like was that I can only get it to 24-25 mph gps, and my boat weighs about 500 lbs right now. My 40 hp swinging a 10x17 on my 525 lb 15' gets 31 mph. Im not a speed demon, but I was hoping to get around the 30 mph mark. Would lowering the diameter but increasing pitch help? I know its hard to tell without the rpms, but until I get one installed I though the gurus on here might be able to guess.

 

[steelespike]

Re: 1967 Mercury 500 prop question

It appears your diameter will be about 13"And you may find something marginally smaller
not likely do to the spline count. Do to your probable gear ratio I think you need the diameter. Very likely you need a lower pitch
but not till we get rpm and speed.
I took a look for 15 spline props it appears that some 65 hp have a 15 spline and diameters
a little over 13 and sometime 14" As pitch goes down diameter goes up on most props.
You'll have to measure to see if a 14" will fit. That also may help tell what lower you have.

 

[1973Chieftain]

Re: 1967 Mercury 500 prop question

So you are guessing that actually I might need to find something smaller pitched, perhaps along the lines of a 13"x11"? I'll research how to install a tiny tach and put that towards the front of the list of things to buy!! You are right, my prop options are limited to larger diameter props due to the 15 tooth spline.. Iboats does have a 14"x11" but nothing smaller in diameter. How much does the diameter effect it vs the pitch?

 

[steelespike]

Re: 1967 Mercury 500 prop question

The prop should perform like a 11" prop regardless of the diameter though when going from a 13 X 13 to a 14 X 11
it may affect rpm a little. Keep in mind that normally a 13" pitch prop would be over 13" in diameter. I believe
your prop is an exception.
I also arbitrarily picked a 65 hp merc prop application because it had 15 splines and some props were around 13" in diameter.
There could be other mercs with 15 splines that might have smaller diameters but to this point I haven't found any.